How much of a rush is rush, and who else can do it? How much business (or sleep) will you be losing to meet their deadline? I'm not aware of any data nor standard -- I think it's pretty much what the market will bear...

I thought your question was interesting, so did a little research; you might find these useful:
http://forum.freelanceswitch.com/topic.php?id=6602
http://osxdaily.com/2010/09/07/iphone-development-costs/

One other thought: I think I probably would charge a premium, just so it is clear to the client that I don't normally work this way. Plus, I want to give him the impression (true or not) that I've got so much business that I'm going to have to scramble to put him first. Finally, I'd remind him that he's already getting a bargain rate from me, compared to the big shops out there -- so, a 15% or 25% uptick for a rush still leaves him with a price he'll not easily match elsewhere.